Power-on reset circuit

ABSTRACT

A power-on reset circuit has a first impedance device, a first switch device, a first capacitor, a second switch device, a third switch device, a fourth switch device, a second impedance device, a second capacitor, and a control circuit coupled to a reset input terminal of a circuit device. When the power-on reset circuit is supplied with electric power, the first switch device and the third switch device are turned on and the second switch device and the fourth switch device are turned off. When the electric power is removed from the power-on reset circuit, the first switch device and the third switch device are turned off and the second switch device and the fourth switch device are turned on. The second capacitor is discharged through the fourth switch to a voltage level being close to a ground voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of Taiwan Patent Application No. 101146523, filed on Dec. 11, 2012, the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power-on reset circuit, and in particular to a power-on reset circuit for outputting a predetermined reset signal.

2. Description of the Related Art

When a power source is initially activated, a voltage generated from the power source can not immediately reach a predetermined voltage value. If the voltage which does not reach the predetermined voltage value is applied to a digital circuit, it may cause the initial situation of the digital circuit to be unknown. Generally, the power-on reset circuit is integrated in Very Large Scale Integration (VLSI) circuits for generating a reset signal. Therefore, when the power source is initially activated, the internal circuit of VLSI circuit can obtain a known initial situation according to the reset signal.

FIG. 1 is a diagram illustrating the power-on reset circuit applied to a circuit device. In FIG. 1, when a circuit device 150 is supplied with electric power, the voltage of a Power node V_(DD) coupled to a power-on reset circuit 100 increases from a ground voltage to a predetermined voltage value, such that the power node V_(DD) charges a capacitor C_(P) through a resistor P₁ composed of a PMOS transistor. A control circuit 130 generates a reset signal to the circuit device 150 according to the voltage of the capacitor C_(P). When the electric power is removed from the power node V_(DD), the capacitor C_(P) discharges its power to the power node V_(DD) (at this time, the voltage of the power node V_(DD) is close to the ground voltage) through the resistor P₁. However, because the resistor P₁ is composed of PMOS transistor, the voltage of the capacitor C_(P) can only be dropped to a threshold voltage of the P-N junction in a short time. That is to say, the capacitor C_(P) can not be discharged completely. The voltage of the capacitor C_(P) can not be dropped to a voltage value equal to the reference ground GND. Therefore, when the circuit device 150 (power-on reset circuit 100) is supplied with electric power the next time, the voltage that is preexisted in the capacitor C_(P) drives the control circuit 130 to generate an incorrect control signal, which would further incapacitate the power-on reset circuit 100.

Therefore, a new power-on reset circuit is needed. During the discharging operation, the voltage of the capacitor of the power-on reset circuit is able to be dropped to a voltage value equal to the ground voltage. Thus, the next time the power-on reset circuit is supplied with electric power, the control circuit of the power-on reset circuit can generate the correct control signal, or enable the power-on reset circuit to work normally.

BRIEF SUMMARY OF THE INVENTION

The present invention presents a power-on reset circuit, comprising a first impedance device and a first switch device, each of which is connected to each other and coupled between a power node and a first node. A control terminal of the first switch device is coupled to the power node. The power-on reset circuit further comprises a first capacitor coupled between the first node and a reference ground, and a second switch device coupled between the first node and a connection node. A control terminal of the second switch device is coupled to the power node. The power-on reset circuit further comprises a third switch device coupled between the connection node and the reference ground. A control terminal of the third switch device is coupled to the power node. The power-on reset circuit further comprises a fourth switch device coupled between a second node and the reference ground. A control terminal of the fourth switch device is coupled to the connection node. The power-on reset circuit further comprises a second impedance device coupled between the power node and the second node, and a second capacitor coupled between the second node and the reference ground. The power-on reset circuit further comprises a control circuit coupled between the second node and a reset input terminal of a circuit device. When the power node is supplied with electric power, the first switch device and the third switch device are turned on and the second switch device and the fourth switch device are turned off. Under this condition, the electric power supplied to the power node charges the second capacitor through the second impedance device. When the voltage of the second capacitor is elevated to a predetermined voltage value, the control circuit outputs a reset signal according to the predetermined voltage value. When the electric power is removed from the power node, the first switch device and the third switch device are turned off and the second switch device and the fourth switch device are turned on. Under this condition, the second capacitor is discharged to a voltage level being close to the reference ground through the fourth switch.

A detailed description is given in the following embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating the power-on reset circuit applied to a circuit device;

FIG. 2A is a diagram illustrating the power-on reset circuit applied to a circuit device according to an embodiment of the invention;

FIG. 2B is a diagram illustrating the power node supplied with electric power in FIG. 2A;

FIG. 2C is a diagram illustrating that the electric power is removed from the power node in FIG. 2B;

FIG. 3 is a diagram illustrating a power-on reset circuit applied to a circuit device according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

FIG. 2A is a diagram illustrating the power-on reset circuit applied to a circuit device according to an embodiment of the invention. A power-on reset circuit 200 comprises: a first impedance device R₁, a second impedance device R₂, a first switch device SW₁, a second switch device SW₂, a third switch device SW₃, a first switch device SW₄, a first capacitor C₁, a second capacitor C₂, and a control circuit 230.

The first impedance device R₁ and the first switch device SW₁ are connected in series with each other and coupled between a power node N_(P) and a first node N₁, and a control terminal T₁ of the first switch device SW₁ is coupled to the power node N_(P). The first capacitor C₁ is coupled between the first node N₁ and a reference ground GND. The second switch device SW₂ is coupled between the first node N₁ and a connection node N_(C) and a control terminal T₂ of the second switch device SW₂ is coupled to the power node N_(P). The third switch device SW₃ is coupled between the connection node N_(C) and the reference ground GND and a control terminal T₃ of the third switch device SW₃ is coupled to the power node N_(P). The fourth switch device SW₄ is coupled between a second node N₂ and the reference ground GND and a control terminal T₄ of the fourth switch device SW₄ is coupled to the connection node N_(C). The second impedance device R₂ is coupled between the power node N_(P) and the second node N₂. The second capacitor C₂ is coupled between the second node N₂ and the reference ground GND. The control circuit 230 is coupled between the second node N₂ and a reset input terminal IN₁ of a circuit device 250.

FIG. 2B is a diagram illustrating that the power node is supplied with electric power in FIG. 2A. When the power node N_(P) is supplied with the electric power (a voltage V_(DD) is applied to the power node N_(P)), the first switch device SW₁ is turned on. The electric power (voltage V_(DD)) of the power node N_(P) charges the first capacitor C₁ through the first impedance device R₁. At the same time, the electric power of the power node N_(P) manipulates the second switch device SW₂ to turn off and the third switch device SW₃ to turn on. Because the third switch device SW₃ is turned on, the voltage of the connection node N_(C) is close to the reference ground GND. Thus, the fourth switch device SW₄ is turned off. The electric power of the power node N_(P) charges the second capacitor C₂ through the second impedance device R₂. When the electric power of the power node N_(P) charges the second capacitor C₂ to a predetermined voltage value, the control circuit 230 outputs a reset signal to the reset input terminal IN₁ of the circuit device 250 according to the predetermined voltage value.

FIG. 2C is a diagram illustrating that the electric power is removed from the power node in FIG. 2B. When the electric power is removed from the power node N_(P) (for example, the power node N_(P) is coupled to the reference ground GND), the first switch device SW₁ and the third switch device SW₃ are turned off and the second switch device SW₂ is turned on. The voltage of the first capacitor C₁ is transmitted to the control terminal T₄ of the fourth switch device SW₄ through the second switch device SW₂, so as to turn on the fourth switch device SW₄. The second capacitor C₂ is discharged to a voltage level being close to the reference ground GND through the fourth switch device SW₄.

When the electric power is removed from the power node N_(P), the voltage required to operate the power-on reset circuit 200 is supplied from the first capacitor C₁. Therefore, no additional electric power is available to generate the current flowing into the power-on reset circuit 200, and thus the value of the quiescent current is zero.

FIG. 3 is a diagram illustrating a power-on reset circuit 300 applied to a circuit device according to an embodiment of the invention. The circuit structure in FIG. 3 is the same as FIG. 2, and the difference is that FIG. 3 further discloses the implementation of the circuit in detail. As shown in FIG. 3, the first switch device SW₁, the third switch device SW₃ and the fourth switch device SW₄ shown in FIG. 2 are respectively composed of a NMOS transistor MN₁, MN₂, and MN₄, and the second switch device SW₂ is composed of a PMOS transistor MP₂. Next, as shown in FIG. 3, the first impedance device R₁ and the second impedance device R₂ shown in FIG. 2 are respectively composed of a PMOS transistor MP₁ and MP₃, wherein the gate electrode and the drain electrode of the PMOS transistor MP₁ and MP₃ are connected to each other. It is noteworthy that the first impedance device R₁ and the second impedance device R₂ are respectively composed of a PMOS transistor MP₁ and MP₃ in this embodiment. However, this circuit topology is used only for illustration, but it is not limited thereto. Because the second switch device SW₂ and the third switch device SW₃ are composed of a PMOS transistor MP₂ and a NMOS transistor MN₂, respectively, the second switch device SW₂, is turned off and the third switch device SW₃ is turned on as the power node N_(P) is supplied with electric power. On the contrary, when the power node N_(P) is not supplied with electric power, the second switch device SW₂ is turned on and the third switch device SW₃ is turned off. The circuit operation in FIG. 3 is described in the preceding paragraphs involved with the circuitry of FIG. 2A˜2C, so there is no need to go further on this issue.

Because the second capacitor C₂ is discharged to the voltage level equal to the reference ground GND, the next time that the power node N_(P) of the power-on reset circuit 300 is supplied with electric power, the power-on reset circuit 300 can operate normally and output a reset signal.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A power-on reset circuit, comprising: a first impedance device and a first switch device, each of which is connected in series with each other and coupled between a power node and a first node, wherein a control terminal of the first switch device is coupled to the power node; a first capacitor, coupled between the first node and a reference ground; a second switch device, coupled between the first node and a connection node, wherein a control terminal of the second switch device is coupled to the power node; a third switch device, coupled between the connection node and the reference ground, wherein a control terminal of the third switch device is coupled to the power node; a fourth switch device, coupled between a second node and the reference ground, wherein a control terminal of the fourth switch device is coupled to the connection node; a second impedance device, coupled between the power node and the second node; a second capacitor, coupled between the second node and the reference ground; and a control circuit, coupled between the second node and a reset input terminal of a circuit device; wherein when the power node is supplied with electric power, the first switch device and the third switch device are turned on and the second switch device and the fourth switch device are turned off to allow the second capacitor to be charged with the electric power of the power node through the second impedance device, and the electric power of the power node charges the second capacitor to a predetermined voltage value, the control circuit outputs a reset signal; when the electric power is removed from the power node, the first switch device and the third switch device are turned off and the second switch device and the fourth switch device are turned on, and the second capacitor is discharged to a voltage level being close to the reference ground through the fourth switch device.
 2. The power-on reset circuit as claimed in claim 1, wherein: when the power node is supplied with electric power, the electric power charges the first capacitor; and when the electric power is removed from the power node, the voltage of the first capacitor is supplied to the control terminal of the fourth switch device through the second switch device, so as to turn on the fourth switch device.
 3. The power-on reset circuit as claimed in claim 1, wherein the first switch device, the third switch device, and the fourth switch device are NMOS transistors, and the second switch device is a PMOS transistor. 